netdev - Re: [PATCH bpf-next v4 02/12] net, sk_msg: Annotate lockless access to sk

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Date:   Thu, 23 Jan 2020 11:15:22 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Jakub Sitnicki <jakub@...udflare.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        John Fastabend <john.fastabend@...il.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        kernel-team@...udflare.com,
        John Fastabend <john.fastabend@...il.com>,
        Lorenz Bauer <lmb@...udflare.com>, Martin Lau <kafai@...com>
Subject: Re: [PATCH bpf-next v4 02/12] net, sk_msg: Annotate lockless access
 to sk_prot on clone

Jakub Sitnicki wrote:
> On Thu, Jan 23, 2020 at 06:18 PM CET, Eric Dumazet wrote:
> > On 1/23/20 7:55 AM, Jakub Sitnicki wrote:
> >> sk_msg and ULP frameworks override protocol callbacks pointer in
> >> sk->sk_prot, while tcp accesses it locklessly when cloning the listening
> >> socket, that is with neither sk_lock nor sk_callback_lock held.
> >>
> >> Once we enable use of listening sockets with sockmap (and hence sk_msg),
> >> there will be shared access to sk->sk_prot if socket is getting cloned
> >> while being inserted/deleted to/from the sockmap from another CPU:

[...]

> >>  include/linux/skmsg.h | 3 ++-
> >>  net/core/sock.c       | 5 +++--
> >>  net/ipv4/tcp_bpf.c    | 4 +++-
> >>  net/ipv4/tcp_ulp.c    | 3 ++-
> >>  net/tls/tls_main.c    | 3 ++-
> >>  5 files changed, 12 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
> >> index 41ea1258d15e..55c834a5c25e 100644
> >> --- a/include/linux/skmsg.h
> >> +++ b/include/linux/skmsg.h
> >> @@ -352,7 +352,8 @@ static inline void sk_psock_update_proto(struct sock *sk,
> >>  	psock->saved_write_space = sk->sk_write_space;
> >>
> >>  	psock->sk_proto = sk->sk_prot;
> >> -	sk->sk_prot = ops;
> >> +	/* Pairs with lockless read in sk_clone_lock() */
> >> +	WRITE_ONCE(sk->sk_prot, ops);
> >
> >
> > Note there are dozens of calls like
> >
> > if (sk->sk_prot->handler)
> >     sk->sk_prot->handler(...);
> >
> > Some of them being done lockless.
> >
> > I know it is painful, but presumably we need

Correct.

> >
> > const struct proto *ops = READ_ONCE(sk->sk_prot);
> >
> > if (ops->handler)
> >     ops->handler(....);
> 
> Yikes! That will be quite an audit. Thank you for taking a look.
> 
> Now I think I understand what John had in mind when asking for pushing
> these annotations to the bpf tree as well [0].

Yep this is what I meant. But your patches don't make the situation
any worse its already there.

> 
> Considering these are lacking today, can I do it as a follow up?

In my opinion yes. I think pulling your patches in is OK and I started
doing this conversion already so we can have a fix shortly. I didn't
want to push it into rc7 though so I'll push it next week or into
net-next tree.

.John